Sammo

Answer: I'd probably rule out of the last reason you mentioned, on the first one you have to consider that the lie goes deeper than just the father's identity, if that is a lie at all. Through the information he saw on the medical record - and he kills her after seeing that record - it turns out that she had let him be abused and beaten up since a tender age, which would probably be the cause of his mental and physical issues. In his line before the murder, he mentions her full name, and how he has not been happy a minute of his whole life, so the terrible childhood, which maybe now came back to him, is a real factor. Mentioning the comedy, so a play, might also imply a sort of detachment now that he understands that it was all a lie from the beginning and she's not even his real mother (if that's what he believes).

Sammo ★

Answer: 1 - Why does he dance a bunch of times in the movie? It's kinda his thing! You can see that Murray himself does little goofy dances all the time during his show and Arthur sorta seems to have taken that trait from him. 2 - I have heard that "comedy is subjective" for years and years. It is sort of a truism but it does fit, some people like a certain kind of humor that can fall completely flat for others. I am not sure if you should read some specificity about it in this movie, except for the fact that obviously he's using it to justify his own actions to the point of considering 'funny' to commit murder in the context of the perspective change when he starts considering his tragic life a whole big joke with no punchline. I don't really see in this movie a satire of show business or shock comedy, since it seems to focus on other themes (mental illness, emargination, etcetera).

Sammo ★

Answer: 1. It's supposed to be a highly emotional moment for him, the first time he takes control of his own life. He basically celebrates that. 2. I think he is referring to Murray, or at least something in-universe. This is all of course speculative.

lionhead

Answer: Yeah it's completely up to the viewer to believe he killed her or not. I don't think he did, he liked her, just like Gary. I think he visited to see if it was all in his head, with that confirmed he just left.

lionhead

Answer: Todd Philips actually answered this in an interview on IndieWire; "As the filmmaker and the writer I am saying he doesn't kill her. We like the idea that it's almost like a litmus test for the audience to say, 'How crazy is he?' Most people that I've spoken to think he didn't kill her because they understand the idea that he only kills people that did him wrong. She had nothing to do with it. Most people understood that, even as a villain, he was living by a certain code. Of course he didn't kill this woman down the hall."

Sammo ★